A Polarized Electron PWT Photoinjector David Yu DULY Research Inc. California, USA SPIN2004, Trieste, Italy 10/14/04

The DULY Team David Yu Al Baxter Marty Lundquist Yan Luo Chen Ping Alexei Smirnov *Work supported by US DOE Phase II SBIR

DULY Plane-Wave-Transformer (PWT) Photoinjectors S band (2.856 GHz) PWT fabricated and installed at UCLA X band (8.65 GHz) PWT design S band (2.856 GHz) polarized electron PWT in progress S band (2.998 GHz) high rep rate PWT preliminary design Scaleable to L band (1.3 GHz) for ILC US patent numbers , , , others pending.

PWT Photoinjector Applications High quality (polarized) electron source for nuclear physics Low emittance (polarized) electron source for future linear colliders High brightness, short electron bunches for advanced light sources (FEL, XFEL, synchrotrons) Exotic applications (e.g. Compton x-ray source)

Linear Collider Requirements for a Polarized Electron Source See J. Clendenin talk (SLAC) (Friday pm) High polarization (Strained GaAs photocathode) High peak brightness (Laser intensity, pulse shape and wavelength; emittance compensation) Multi-bunch operation (Low dark current– ultra high vacuum, surface preparation; low gradient) High rf pulse and/or beam bunch rep rate (Effective cooling required to remove heat) L-band SC linacs recommended for ILC

Parameters of an S-Band PWT Polarized Electron Gun Charge per Bunch (nC) Frequency (MHz) 2856 Energy (MeV) Normalized RMS Emittance (mm-mrad, no thermal emit.) Energy Spread (%) Bunch Length (rms, ps) Peak Current (A) Linac Length (cm) 58 Beam Size (rms, mm) Peak Magnetic Field (Gauss) Peak Electric Field (MV/m) Peak Brightness (10 14 A/m 2 -rad 2 )

PWT RF Design SW 10+2/2 cell structure 11 rod-supported disks TEM-like mode outside TM01-like  -mode on axis

PWT Magnetic Design Principle of Emittance Compensation (a la B. Carlsten, J. Rosenzweig and L. Serafini) Focusing solenoids and magnetic flux Longitudinal field vs axial distance

Parmela Beam Dynamics Simulations with and without Matching (Q = 1 nC) Matching TW linac (red bar) Accelerating field = 25 MV/m Injection phase = 0 degree PWT (blue bar) only Peak field=55 MV/m R max =1.8 mm Bunch length=10 ps

Parmela Simulations for Ultra-Short Bunch (Q = 10 pC) Initial beam spot size = 2 mm, bunch length = 30 fs At beam waist, spot size=0.4 mm, bunch length=50 fs

 Cathode  1 st Iris from first iris to cathode rf peak field=55 MV/m, 90 deg from cathode holder to 1 st iris rf peak field=22.5 MV/m, 90 deg Energy Gain Secondary Electrons Backstreaming

Operating the PWT at a low field gradient helps prevent backstreaming electrons emitted from the first PWT iris from reaching the photocathode. Inset shows half iris geometry. Dark Current Suppression at Low Field Gradient

Thermal Hydraulic Design (S-band PWT) NLC parameters: 120-Hz, 3 microsecond-long rf pulses, 50-MW peak power at S band ⇒ 18 kW total. In a PWT structure with copper disks and rods and SS tank, 29.9% goes into the 11 disks (489 W per disk) and 11.4% into the 4 rods (186 W per section between disks). Heat in disk and rod (0.18” ID) is removed by water flow through hollow rods and disk internal channels. Required flow in each disk is 5.2 lpm at  T=9°C. Use 2 parallel cooling circuits (for 5 and 6 disks respectively), with a variable orifice size (0.06” to 0.18”) for each disk to account for line pressure drop. Required external pressure head is 323 psi for a total flow of 30.9 lpm through the inlet/outlet pipes.

Hydraulic test for a single disk

PWT Disk Thermal Study Microcomputer measurements Cooling fluid flow rate Temperature monitoring Closed loop temperature generation

Ultra High Vacuum Design Block diagram of major components of the PWT load lock Isometric view of polarized electron PWT gun with load lock Activated GaAs photocathode vacuum < – Torr requires ultra high

Pumping box with Conflat design (left) and Helicoflex design (right) Close-up of the PWT tank section inside the pumping box Achievable vacuum is better than Torr at the cathode. 2 NEG pumps and 1 ion pump in the PWT provide a total pumping capacity > 1200 liters/sec. Large vacuum conductance is only limited by the perforated section of PWT tank inside the pumping box.

Load-Lock System for Activated GaAs Photocathode Ultra high vacuum transport Ultra high vacuum processing Proprietary puck handling

An L-Band PWT Gun as an ILC Polarized e – Injector? L-band PWT polarized injector to the SC linac Can be replaced by:

Thermal Hydraulic Design (L-band PWT) ILC parameters: 5-Hz, 1370 microsecond-long rf pulses, 10-MW peak power at L band ⇒ 68.5 kW total. In a PWT structure with copper disks and rods and SS tank, 30% goes into the 6 disks (3.4 kW per disk) and 11% into the 4 rods (1.3 kW per section between disks). Heat in disk and rod (0.43” ID) is removed by water flow through hollow rods and disk internal channels. Required flow in each disk is 48.8 lpm at  T=9°C. Use 2 parallel cooling circuits (3 disks each), with a variable orifice size (0.28” to 0.40”) for each disk to account for line pressure drop. Required external pressure head is 106 psi for a total flow of 146 lpm through the inlet/outlet pipes.

Conclusions Electromagnetic, thermal hydraulic, vacuum and mechanical designs have been performed for an S-band polarized electron PWT photoinjector. Polarization > 85%, charge/bunch up to 2 nC, energy gain ≈ 18 MeV, normalized transverse emittance ≈1 mm-mrad. An S-band PWT is suitable for nuclear physics and HEP experiments. Ultra short bunches are available for FEL and other applications. An L-band polarized electron PWT photoinjector offers low transverse emittance and excellent cooling for the International Linear Collider (ILC).